Method and apparatus for safety testing optical systems for hazardous locations

ABSTRACT

A method of determining the ignition characteristics of an optical source emitting optical power into a hazardous environment includes providing a chamber and a tapered optical fiber having an input end and an output end, wherein the output end has a smaller diameter than the input end. The output end of the tapered fiber is disposed within the chamber and the input end of the tapered fiber is optically coupled to the optical source for receiving optical power therefrom. Power is first applied to the tapered fiber and the power output at the tapered fiber output end measured. Then a target is applied to the tapered fiber output end, and the chamber is filled with the desired gas/air mixture and the same power applied to the tapered fiber. After power is applied for a period of time, a determination is made whether or not the gas/air mixture ignited.

PRIORITY CLAIM

This is a § 371 U.S. national stage of PCT/US00/09571, filed Apr. 10,2000, which was published under PCT Article 21(2), and claims thebenefit of U.S. application No. 60/129,470, filed Apr. 15, 1999.

FIELD OF THE INVENTION

This invention relates to a method and apparatus for testing opticalcomponents located in hazardous environments for the possibility ofignition and/or burning. The method and apparatus of the invention willallow testing laboratories to efficiently and accurately vary theirradiance of optical sources for certifying the safety of opticalsystems in hazardous (including classified) locations.

BACKGROUND OF THE INVENTION

Fiber optic systems containing intense laser sources, such as laserdiodes capable of producing several hundred milliwatts or more of power,are found in numerous industrial measurement, monitoring, and controlapplications. Besides the risk of human exposure, one safety concernwith intense radiation sources is the potential for ignition offlammable gases, vapors, dusts, fibers, or flyings found in someindustrial and hazardous locations.

Experiments have shown that optical devices, such as lasers, can providesufficient energy to cause ignition of flammable gas/air orparticulate/air mixtures, and, in some situations, burning of flammablematerials present. One ignition process requires the conversion ofoptical energy to thermal energy by absorption in an appropriate targetas shown in FIG. 15. Fiber optic systems placed within hazardousenvironments (i.e., areas in which there is a chance of explosion due toincreased temperatures or flammable combinations of flammable gas/air orparticulate/air mixtures) can be a potential ignition source. Faultyoptical equipment, broken or stretched fiber optic cable, or improperlyinstalled optical equipment can output optical beams above the criticalor flash point in hazardous locations.

One particular application of optical technologies is the remotemeasurement of explosive methane gas in underground coal mines. Methanegas is often liberated during the mining process. In addition tomethane-air mixtures, coal dust suspensions in air represent anexplosion hazard and larger accumulations of coal dust on surfacesrepresent a smoldering fire hazard. Federal regulations require periodicmethane measurements at the mining face, and abatement measures must betaken when methane concentrations exceed a threshold. Methanemeasurements often require elaborate safety precautions to preventinjury in the roof-fall prone face area. The difficulty in making remotemethane measurements during extended cut operations has been cited as asafety concern by the United Mine Workers of America.

A remote measurement procedure has been proposed (see FIG. 13) in whichan open laser beam passes through an area where both methane gas andcoal dust are normally present. Federal regulations require thatatmospheric monitoring systems used in gassy underground mines shall beintrinsically safe. However, intrinsic safety applies primarily topreventing electrical sparks or electrical heating ignition mechanisms.Little or no consideration has been given to optical ignition mechanismsbecause typical lasers used in mines have very low power. Also, the MineSafety and Health Administration (MSHA) criteria for the evaluation andtest of intrinsically safe apparatus and associated apparatus contain nospecific guidance for optoelectronic components such as laser diodes.

One way to ensure the safety of a remote optical monitor is to limit theenergy of the laser beam to below the critical duration and intensitywhich will result in ignition or burning in the proposed environment.Previous techniques to determine the duration and intensity needed foran optical component for ignition studies used external opticalcomponents to vary the irradiance (the optical power per area or opticalpower density) of the optical sources. Five cooperating laboratories inEurope investigated the conditions under which optical instruments usingintense light sources (such as lasers) could operate safely in hazardousatmospheres containing vapors of various combustible products and/orcombustible particulates. This study investigated the nature of light(i.e., coherence, intensity, wavelength, spectral width, andmodulation), the characteristics of the illuminated particles (i.e.,size, chemical and physical nature), and the nature of the gaseousenvironment. This study concluded that continuous wave devices radiatingin the visible and near visible are not hazardous provided either theradiated power is less than 35 milliwatts, or the peak radiation flux isless than 5 milliwatts per square millimeter.

Details the these experimental techniques employed by the fivecooperating laboratories is described in the report, “Optical Techniquesin Industrial Measurement: Safety in Hazardous Environments,” EuropeanCommission, EUR 16011 EN, 1994. However, these techniques required verycareful alignment and typically were designed for a single type opticalsource and were not easily adaptable to different optical sources.Accurately verifying the spot size of the output beam was alsodifficult, resulting in a time consuming test setup and the need for avariety of components (including different lenses and optical fibers) onhand to accommodate different optical sources. Also, the power valuesconcluded to be safe may not be sufficient to provide remote monitoringin applications such as remote monitoring of methane in a miningoperation.

There is a need for a method of testing optical systems, especially wheninstalled in hazardous locations, to determine the risk of ignition.There is a need for a method of determining the maximum safe poweroutput of an optical system to avoid the risk of ignition in a hazardouslocation. There is also a need for a method of determining the maximumsafe power output of an optical system to avoid the risk of burning inhazardous locations where flammable materials are present. There is aneed for an apparatus or system for testing optical systems for risk ofignition and burning which is easy to use with different opticalsources. There is also a need for an optical test apparatus or systemwhich provides for precise adjustment and verification of the outputbeam spot size. There is a need for an optical test apparatus or systemwhich does not require use of additional components to accommodatedifferent optical sources. There is also a need for a method to evaluatethe failure mode when an optical fiber is stretched to the breakingpoint, with a concurrent reduction in fiber diameter, increasing theirradiance of escaping laser power. The present invention provides suchmethods and systems.

SUMMARY OF THE INVENTION

A method of determining the ignition characteristics of an opticalsource emitting optical power into a hazardous environment according tothe invention includes providing a chamber and a tapered optical fiberhaving an input end and an output end, wherein the output end has asmaller diameter than the input end. The output end of the tapered fiberis disposed within the chamber and the input end of the tapered fiber isoptically coupled to the optical source for receiving optical powertherefrom. Power is first applied to the tapered fiber and the poweroutput at the tapered fiber output end measured. Then a target isapplied to the tapered fiber output end, and the chamber is filled withthe desired gas/air mixture and the same power applied to the taperedfiber. After power is applied for a period of time, a determination ismade whether or not the gas/air mixture ignited.

Apparatus for determining the ignition characteristics of an opticalsource emitting optical power into a hazardous environment according tothe invention includes a chamber for receiving a known quantity of ahazardous material. A tapered optical fiber having an input end and anoutput end, wherein the output end has a smaller diameter than the inputend, wherein the output end is disposed within the chamber. An opticalcoupler optically couples the tapered fiber input end to the opticalsource for receiving optical power therefrom. A target is attached tothe output end of the tapered fiber and a video camera or pressuresensor is used to determine if ignition occurs when power is applied tothe tapered fiber in the gas/air mixture filled chamber.

A method of determining the smoldering or burning characteristics of anoptical source emitting optical power into a hazardous environmentaccording to the invention includes measuring the temperature of thetarget at the tapered end of the fiber optic taper as power is appliedto the taper.

The concept of conservation of brightness states that if light lossesare negligible, the spatial and angular content of the light anywherewithin or at either end of a fiber optic taper are described by:

S _(i) n _(i) ² sin²(θ_(i))=S _(o) n _(o) ² sin²(θ_(o))

where subscript i refers to input parameters, subscript o refers tooutput parameters, S is the cross-sectional area of the lightdistribution normal to the taper axis, θ is the maximum angular extentof the light distribution, and n is the refractive index of the mediumwhere θ is measured. Since n sin(θ) is defined as the numerical aperture(NA) of the fiber and since S_(i)/S_(o)=R², where R² is the taperdiameter ratio, then NA_(o)/NA_(i)=R.

These expressions show that fiber optic tapers are useful fortransforming spatially structured input beams (such as those produced bydifferent optical sources) into a spatially uniform output spot.Therefore, a single taper can be used to more efficiently guide opticalenergy from a larger variety of input sources while maintaining uniformoutput characteristics than possible from a non-tapered fiber. Theuniform output characteristics and well-defined taper dimensions allowsmore accurate measurement of irradiance than practical with the externaloptical components used by the prior art. Accurate measurements arecritical in the safety certification process. Also, by transforming thebeam within the taper, external components are not necessary to adjustthe power or focus the output spot, simplifying the test setup, reducingcomponent inventory, and reducing the risk of human exposure to theoptical beam.

These expressions also show (and as supported by experimental evidenceobtained by the inventors) that a fiber optic taper increases theirradiance of an optical source. Thus, a method and apparatus employinga taper allows a controlled way of simulating enhanced irradiance thatmay occur in a broken optical fiber.

Beam transformation within the fiber eliminates the need for externaloptical components. Fiber tapers require less careful alignment thanexternal components, simplifying the test setup and reducing setup time.Fiber tapers can accept a wide range of optical sources whilemaintaining uniform output characteristics, reducing component inventoryrequired if using external components. The well-defined outputcharacteristics of the method of the invention will allow accuratemeasurement of optical beam properties more easily and reliably thanpractical with external components.

The method and apparatus of the invention will allow testinglaboratories to efficiently and accurately vary the irradiance ofoptical sources for certifying the safety of optical systems inhazardous (including classified) locations. The irradiance enhancementdemonstrated by the method of the invention will allow a controlledmethod for simulating potential irradiance enhancement of a brokenoptical fiber. The smaller cross-sectional area of the taper reducesthermal conductivity effects, representing more severe testingconditions of fiber-optical systems than using untapered fibers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-section of an embodiment of the invention;

FIG. 2 shows dimensional characteristics of the fiber taper shown inFIG. 1;

FIG. 3 is a graph of absorbance of black iron oxide, Pittsburgh coal,and limestone rock dust samples;

FIG. 4 is a graph of minimum igniting powers with selected targetsplaced on a 400 μm core optical fiber placed in flammable methane-airmixtures;

FIG. 5 is a graph of igniting powers for optical fibers tipped withPittsburgh coal (3 μm)-Krytox mixture, placed in flammable methane-airmixture;

FIG. 6 is a graph of igniting power versus coal particle size foroptical fiber tipped with fiber sized Pittsburgh coal cyanoacrylatetargets, 8% methane in air;

FIG. 7 is a graph of igniting powers for optical fibers tipped with ironoxide-Krytox placed in methane-air mixtures;

FIG. 8 is a graph of igniting power versus lag time for various targetsplaced in flammable methane-air mixtures;

FIG. 9 is a graph of igniting power versus fiber optic core diameter forvarious targets;

FIG. 10 is a graph of laser diode electrical input and CW optical powercharacteristics;

FIG. 11 is a graph of MSHA accepted ignition curve for resistivecircuits;

FIG. 12 is a plot of minimum laser power for ignition for variousignition temperatures, compared to the experimental data for iron oxidefrom FIG. 9;

FIG. 13 is a schematic diagram of an open-beam, remote monitor;

FIG. 14 is an alternate embodiment of the invention; and

FIG. 15 is a schematic description of the ignition process for theconversion of optical energy to thermal energy by absorption in anappropriate target.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

While the invention can be used with many different optical sources tosimulate many different hazardous locations, for convenience, theinvention will be described with respect to an apparatus and method foruse in a coal mining location where the presence of methane must bemonitored. The primary mode of ignition for methane-air atmospheres atpower levels common in current measurement and control applicationsrequires the simultaneous presence of a flammable methane-airatmosphere, a radiating energy source of the duration and intensityneeded to cause ignition, and an appropriate target to convert theoptical energy to thermal energy.

Ignition requires the conversion of optical energy to thermal energy byabsorption in an appropriate target. The target needs to attain aminimum ignition temperature for a given ignition volume in order toignite the surrounding gas. Some relevant target properties includeabsorbance, surface area, volatility, and reactivity with air. It isgenerally believed that strongly absorbing targets facilitate ignition,but the effect of target surface area, volatility, and reactivity isless clear. For example, small, volatile or combustible targets mayvaporize, dissipating the laser energy before igniting the surroundinggas. Larger combustible targets may have sufficient mass to contributesignificant heat of combustion to ignite methane-air mixtures moreeasily than a similarly sized inert target. Also, larger heated targetscan ignite methane-air mixtures at lower temperatures than smallertargets, but require higher incident powers to attain similartemperatures as small targets. Small targets that vaporize nearappropriate ignition temperatures may ignite gases more readily thanother small targets by achieving a minimum volume.

Referring now to the drawings and especially to FIG. 1, an apparatus fortesting an optical source is generally shown therein and identified byreference numeral 10. Apparatus 10 includes test chamber 13 having aviewing port 22 to which a video camera may be attached for viewing andrecording the ignition tests. Preferably the test chamber 13 has a20-liter interior chamber 18 suitable for explosion testing of dusts,gases, and their mixtures. The chamber 13 can also be used to measurelean and rich limits of flammability, explosion pressures, rates ofpressure rise, minimum ignition energies, minimum oxygen concentrationsfor flammability, and amounts of inhibitor necessary to preventexplosions.

A test methane/air mixture is introduced into the chamber through inlet19. A fiber optic taper probe 20 is inserted through opening 17 ofchamber 13 and disposed within the interior 18. Optical energy fromoptical source 11 is coupled to taper 20 via coupler 12, which ispreferably a bulkhead adapter. A target 14 is disposed at the output endof taper 20. The fiber optic taper 20 directs optical energy from theoptical source under test 11 into chamber interior 18. Referring to FIG.2, optical energy launched into the input end 34 of fiber optic taper 20is tapered down to a small fiber diameter 35 and guided to the outputend 36 (where the target is attached, but not shown here) in theignition chamber 18. Preferably, taper 20 tapers from 400 μm at inputend 34 to 200 μm at diameter 35 and at output end 36.

Optical source 11 is preferably a SDL model 8110-B Integrated LaserSystem (ILS). The ILS output power is variable up to 10 watts out of a400 μm diameter aperture. The laser diode wavelength is centered at 803nm in the near infrared. The ILS was operated in constant power modewhich eliminated overshoot, and produced a 100 millisecond rise time.The ILS also contains a low power visible aiming laser which is usefulfor setting up experiments.

Three sizes of fiber optic cable were used. In addition to the fiberoptic taper 20, which is preferably a taper from 400 μm to 200 μm,non-tapered cable was used for comparison. Commercially availableFiberguide Anhydroguide plastic clad silica (PCS) 400 μm to 200 μm fiberoptic taper, Spectran 400 μm core, 430 μm clad Hard Clad Silica cable,0.4 numerical aperture (NA), and Fiberguide Anhydroguide PCS, 800 μmcore, 900 μm clad diameter, 0.4 NA cable were compared.

Selected targets 14 included Pittsburgh seam coal (PC) and black ironoxide. Black iron oxide (a combination of ferrous FeO and ferric Fe₂O₃oxide having a theoretical formula of Fe₃O₄) was chosen because of itsexcellent optical absorption and inertness. Particle size is uniformwith an average diameter of approximately 0.4 μm. Pittsburgh seam coalis used in standardized MSHA dust blanketing tests for intrinsic safetyevaluations. The MSHA standardized tests call for dust fine enough topass through a 200 mesh (75 μm) screen. Very fine PC particles with amass median diameter of 3 μm were used in a series of tests to comparewith the iron oxide results. For ignition tests with fine particles, acollection of many particles was placed on the fiber-optic tip 36.Larger individual coal particles approximately the size of thefiber-optic core diameter were used in another series of tests toinvestigate potential heat of combustion from larger coal particles.

Absorption characteristics were determined from a sample of iron oxide,200 mesh PC and limestone rock dust (CaCO₃). Referring to FIG. 3, it canbe seen that black iron oxide is a slightly stronger absorber than coalover the wavelengths measured. Both are much stronger absorbers ofradiation than limestone rock dust, a material commonly applied inunderground coal mines to prevent coal dust explosions.

Before each ignition test, the power emanating from output tip 36 wasmeasured using a laser power meter (Scientec Model D200PC) with attachedcalorimeter (Scientec Model AC2501). This power measurement was taken asthe total power absorbed by the target 14 for the ensuing ignition test.The laser source 11 was then turned off and a test target 14 attached tothe output tip 36. Excess fiber was pulled back through opening 17 untiltarget 14 was positioned near the center of the chamber interior 18 atopa vertically aligned fiber (not shown) which was used to support thetarget. The visible low power aiming laser was used to verify that thetarget completely covered the tip 36.

Chamber 13 was sealed and evacuated and a flammable gas-air mixtureintroduced into chamber interior 18 through port 19. The laser waspowered to the measured power level and ignition (if present) recordedby the video camera. An internal pressure transducer (not shown) wasalso used to sense ignition. Targets 14 were heated to incandescence inall tests whether or not an ignition was produced. Tests were determinedto be non-ignitions and terminated after the video camera showed theintensity of incandescence dropped considerably or ceased. In mostcases, tests were terminated within about one minute after turning onthe laser 11. The flammability of the gas-air mixture was periodicallyverified using electric matches when experiments resulted innon-ignitions. The primary criterion for ignition was the visualappearance of flame on the video. Ignition was also confirmed by theexplosion over pressures, which were about 55 to 100 psi for 6-10%methane-air mixtures. Peak pressures were from 2 to 5 psi for 5%methane-air ignition.

Various methods were used to attach targets 14 to the fiber tip 36. Forexample, a sample of iron oxide particles was mixed with isopropylalcohol, applied to the tip of the fiber until the aiming laser was nolonger visible, and allowed to dry before sealing and evacuating thechamber 18. Mixing the very fine particles with an inert lubricant,Krytox, provided better adhesion and the lowest igniting powers. Krytoxis a fluorinated lubricant that has good temperature stability (lowoutgassing up to about 355° C.) and is nonflammable. The Krytox toparticle ratio of the target mixture was about 1 to 3 by volume.Adhesives such as cyanoacrylate were not used extensively with very fineparticles because of the potential heat of combustion contributions. Thefiber-sized coal particles required an adhesive to adhere adequately tothe fiber tip 36, so cyanoacrylates were used in those cases. Acomparison of minimum igniting powers of various targets on a 400 μmfiber is shown in FIG. 4.

Experimental results are shown in FIGS. 5-7. In each series of testswith a fiber of a particular diameter, the methane concentration wasvaried to find the minimum igniting laser power. In general except forthe fiber-sized coal particles shown in FIG. 6, each set of tests at aparticular methane-air concentration was discontinued after threenon-ignitions were obtained. Additional tests were conducted afterobtaining four non-ignitions in one case because of irregular shapes andreflective (glossy) facets on the larger particles made it difficult toblock the aiming laser. Minimum igniting powers for PC (3 μm) Krytoxtargets (see FIG. 5) were 0.9 watts for the 200 μm core taper fiber and1.5 watts for the 400 μm core non-taper fiber. Minimum igniting powersfor fiber-sized PC-cyanoacrylate targets in 8% methane-air mixtures (seeFIG. 6) were 1.6 watts for 400 μm core non-taper fiber and 2.7 watts for800 μm core non-taper fiber.

Minimum igniting powers (see FIG. 7) with iron oxide Krytox targets were0.6 watts, 1.1 watts and 2.2 watts with the 200 μm core taper, 400 μmcore non-taper and 800 μm core non-taper fibers, respectively. Therelatively flat response with methane concentration resembles autogenousignition temperature (AIT) phenomena more than electrical spark (MIE)phenomena. Limiting thermal phenomena such as AIT are also characterizedby large ignition lag times. Lag time trend is shown in FIG. 8. Ignitionlag times were estimated by observing video tape recorded by the highspeed camera system. Ignition lag was taken as the time between thefirst noticeable target glow and first noticeable flame front emanatingfrom the target. In several cases, barely discernable flame frontsemanating from the target were followed by clearly visible flamesappearing from other portions of the chamber 18.

A summary of minimum igniting powers versus core diameter is shown inFIG. 9. This graph shows that inert but more strongly absorbing ironoxide-Krytox targets consistently ignited methane-air mixtures at lowerpowers than coal targets in this study. Minimum igniting power densitiesfor iron oxide-Krytox targets calculated by dividing the igniting powerby the surface area of the fiber core produces values of 19.2, 8.7 and4.4 watts per square millimeter for the 200 μm taper, 400 μm and 800 μmfibers, respectively. Comparing these calculations to FIG. 9 shows thatsmaller core fibers required lower incident powers for ignition thanlarger core fibers, but larger power densities.

Experimental approaches to assessing minimum igniting phenomena requirea large number of tests to account for statistical variations in testconditions. The number of non-ignitions per test series in FIGS. 5-7 isroughly 10. In comparison, MSHA tests each electrical circuit for 1000revolutions in a spark test apparatus, with multiple sparks for eachrevolution, resulting in at least 5000 make-break sparks. For thisreason, a conservative safety factor should be applied to the curve inFIG. 9. Results suggest that larger core fibers are significantly lesslikely to cause ignition in methane-air mixtures, under certain testconditions. The likelihood of significant intensity fluctuations inmultimode optcial fibers from modal variations or focusing effects frombroken fibers, for example, may need to be considered where appropriate.Test results also show that tapered fibers produce lower igniting powersand approach limiting ignition lag times for quickly than untaperedfibers. Thus, fiber optic tapers are useful for evaluating the failuremode where a multimode optical fiber is stretched to the breaking pointwith a concurrent reduction in fiber diameter at the breaking point.

FIG. 10 shows voltage, current and CW optical power characteristics fora commercial laser diode. The power is measured out of a 100 μm fiberoptic pigtail. FIG. 11 shows the MSHA accepted electrical spark ignitioncurve for resistive circuits, plotting short circuit current versus opencircuit voltage. Even at maximum drive current producing upwards of 600milliwatts optical power, the laser diode drive circuit could be wellwithin the safe boundary from an electrical spark point of view (belowand to the left of the ignition curve). Considering also the opticalconversion efficiency is less than 40%, the laser diode and drivecircuit might be considered safe without further evaluation. However,600 milliwatts optical power out of a 100 μm core diameter fiber isabove the ignition curve of FIG. 9, indicating operation at a lowerpower may be prudent.

Larger coal particles required higher incident powers to ignite 8%methane-air mixtures in this study (FIG. 6), indicating heat ofcombustion contributions were negligible (coal particles were heated towhite-hot incandescence in all tests). This does not necessarily applyto situations where much larger accumulations of coal dust may ignite.

To measure the effect of optical power on temperature of largeraccumulations of coal dust, such as might cause smoldering or burning,an alternative embodiment of the invention may be used. Referring now toFIG. 14, an alternate embodiment of the invention is generally showntherein and identified by reference numeral 100. In this embodiment,apparatus 100 is used to measure the temperature of targets on the endsof optical fibers. Temperature information is used to determine the riskof smoldering fire hazards on accumulations of coal dust on surfaces.Infrared (IR) camera 110 focuses on target 114, attached to the outputend of fiber taper 120, which is coupled via optical coupler 122 tooptical source 124.

Preferably IR camera 110 is an Agema 550 thermal imaging camera, whichrecords temperatures of particles 114 on optical fiber tip 116 heated bylaser power from source 124 that produced methane-air ignition. TheAgema 550 camera system measures the IR radiation at wavelengths of 3.6to 5 μm and shows the calculated temperatures as a false color displayon a monitor. The maximum temperature in the area of interest isdisplayed as a numerical value. The IR camera was calibrated up to 1500°C. A 30/80 closeup lens 112 allowed very high spatial resolution. Priorto observations of the optical fiber, the temperature calibration andspatial resolution of the IR camera 110 were confirmed by using smallapertures placed in front of a blackbody source.

To measure the temperatures of targets 114 on the end of optical fiber120, both the IR camera 110 and the optical fiber 120 were positioned onoptical bench 130. The chamber 13 (FIG. 1) was not needed for thesetests. The camera 110 was positioned about 5 cm from the end of thefiber taper 120 coated with an iron oxide target 114. The fiber 120 wastilted approximately 40 degrees from vertical (50 degrees from thecamera axis) to maximize the viewing area of the fiber tip 116.

While there has been illustrated and described a particular embodimentof the present invention, it will be appreciated that numerous changesand modifications will occur to those skilled in the art, and it isintended in the appended claims to cover all those changes andmodifications which followed in the true spirit and scope of the presentinvention.

What is claimed is:
 1. A method for determining a safe power output foran optical source emitting optical power in an environment containinghazardous material, the method comprising: providing a tapered opticalfiber having an input end and an output end, wherein the diameter of theoutput end is smaller than the diameter of the input end; opticallycoupling the optical source to the input end of the optical fiber;applying a predetermined amount of optical power to the optical fiberwith the optical source; measuring the optical power emitted at theoutput end of the optical fiber; and positioning a target to absorboptical power from the output end of the optical fiber.
 2. The method ofclaim 1, further comprising positioning the output end of the opticalfiber and the target in an environment of flammable material anddetecting whether the flammable material is ignited by the target. 3.The method of claim of claim 2, wherein the output end of the opticalfiber and the target are positioned in a chamber containing flammablematerial.
 4. The method of claim 2, further comprising determining theminimum igniting power required to ignite the flammable material.
 5. Themethod of claim 1, wherein the target is disposed on the output end ofthe optical fiber.
 6. The method of claim 1, further comprisingmeasuring the temperature of the target.
 7. The method of claim 1,further comprising detecting whether smoldering of the target occurs. 8.The method of claim 1, wherein the diameter of the input end of theoptical fiber is about 400 μm and the diameter of the output end of theoptical fiber is about 200 μm.
 9. The method of claim 1, wherein theoptical source comprises a laser diode.
 10. The method of claim 2,wherein the flammable material comprises methane.
 11. The method ofclaim 1, wherein the input end and the output end of the optical fiberhave circular cross sections.
 12. A method for determining a safe poweroutput for an optical source emitting optical power in an environmentcontaining hazardous material, the method comprising: providing atapered optical fiber having an input end and an output end, wherein thediameter of the output end is smaller than the diameter of the inputend; optically coupling the optical source to the input of the opticalfiber; applying a predetermined amount of optical power to the input endof the optical fiber with the optical source for which the optical poweremitted at the output end of the optical fiber is known; and positioninga target to absorb optical power from the output end of the opticalfiber.
 13. The method of claim 12, further comprising positioning theoutput end of the optical fiber and the target in an environment offlammable material and detecting whether the flammable material isignited by the target.
 14. The method of claim 13, wherein the outputend of the optical fiber and the target are positioned in a chambercontaining flammable material.
 15. The method of claim 13, furthercomprising determining the minimum igniting power required to ignite theflammable material.
 16. The method of claim 12, wherein the target isdisposed on the output end of the optical fiber.
 17. The method of claim12, further comprising measuring the temperature of the target.
 18. Themethod of claim 17, further comprising detecting whether smoldering ofthe target occurs.
 19. The method of claim 12, wherein the diameter ofthe input end of the optical fiber is about 400 μm and the diameter ofthe output end of the optical fiber is about 200 μm.
 20. The method ofclaim 12, wherein the input end of the optical fiber has a firstcircular cross-sectional area and the output end of the optical fiberhas a second circular cross-sectional area, the second cross-sectionalarea being smaller than the first cross-sectional area.
 21. An apparatusfor determining a safe power output for an optical source emittingoptical power in an environment containing hazardous material, theapparatus comprising: a tapered optical fiber having an input end and anoutput end, the input end having a first diameter, the output end havinga second diameter, the second diameter being smaller than the firstdiameter, the input end being adapted to be optically coupled to theoptical source; a power output measuring apparatus for measuring theoptical power emitted at the output end of the optical fiber; and atarget for placing at a position to absorb optical power emitted fromthe output end of the optical fiber.
 22. The apparatus of claim 21,further comprising an optical coupler for optically coupling the inputend of the optical fiber to the optical source.
 23. The apparatus ofclaim 21, further comprising a chamber for receiving flammable material,and wherein the output end of the optical fiber and the target aredisposed within the chamber.
 24. The apparatus of claim 21, furthercomprising a temperature measuring device for measuring the temperatureof the target when the target is heated by the optical source.
 25. Theapparatus of claim 21, wherein the optical source comprises a laserdiode.
 26. The method of claim 21, wherein the first diameter of theoptical fiber is about 400 μm and the second diameter of the opticalfiber is about 200 μm.
 27. The method of claim 21, wherein the target isdisposed on the output end of the optical fiber.
 28. The method of claim23, wherein the flammable material is methane.
 29. The method of claim21, wherein the input end of the optical fiber has a first circularcross-sectional area and the output end of the optical fiber has asecond circular cross-sectional area, the second cross-sectional areabeing smaller than the first cross-sectional area.